Use describe on new feature tests

The `type: :feature` is automatically detected by RSpec because these
tests are inside the `spec/features` folder. Using `feature` re-adds a
`type: :feature` to these files, which will result in a conflict when we
upgrade to Rails 5.1's system tests.

Because of this change, we also need to change `background` to `before`
or else these tests will fail.

We're also adding a rubocop rule so we dont' accidentally add these
keywords again.

spec/features/admin/change_log_spec.rb

@@ -1,6 +1,6 @@
 require "rails_helper"
 
-feature "Admin change log" do
+describe "Admin change log" do
   let(:budget) {create(:budget)}
   let(:administrator) do
     create(:administrator, user: create(:user, username: "Ana", email: "ana@admins.org"))
@@ -8,7 +8,7 @@ feature "Admin change log" do
 
   context "Investments Participatory Budgets" do
 
-    background do
+    before do
       @admin = create(:administrator)
       login_as(@admin.user)
     end